Abstract

The formation of carbonyl sulphide, OCS, is investigated computationally on a model carbonaceous grain surface (coronene) using density functional theory. Four reaction pathways for the formation of OCS are investigated: formation from CO + S (on both the singlet and the triplet surfaces) and CO + HS, and formation from CS + O (again on both the singlet and the triplet surfaces) and CS + OH. The Langmuir−Hinshelwood, Eley−Rideal, and hot-atom mechanisms are investigated. Calculations show that all species in the ground state are physisorbed on the surface. However, both sulfur and oxygen in their first excited states chemisorb on coronene. The first reaction pathway, 3OCS formation from CO + 3S, is activated by 18.7 kJ mol−1 in the gas phase. This barrier is much too high for the reaction to occur at a significant rate at the low temperatures (10−20 K) found in dark interstellar clouds. However, calculations show that coronene catalyzes this reaction, lowering the barrier to 15.6 kJ mol−1 for the Langmuir−Hinshelwood reaction and to 7.1 kJ mol−1 for the Eley−Rideal reaction compared with the same reaction in the gas phase. For the similar reaction CS + 3O → 3OCS, the gas-phase activation barrier is negative, and it remains so on a coronene surface. The formation of OCS from CO + HS does not take place in a one-step mechanism. Instead, a stable intermediate (HSCO) is formed on the surface, which can subsequently react with a hydrogen atom to form OCS and H2. Finally, CS + OH can react to form a hot HOCS intermediate, which can either react exothermically to yield H + OCS or be stablised on the surface. In the latter case, reaction with another H atom can yield H2 + OCS.